10 Prof, T. Fuchs on the Deep-sea Fauna. 



favourable, as, in fact, the wave-movement produced by 

 violent storms makes itself perceptible, according to the pre- 

 vailing opinion, down to a depth of about 50 fathoms ; and 

 even elsewhere the influence of the motion of the water upon 

 the fauna is not to be denied. But on closer examination the 

 thing seems extremely improbable. For instance, if the lit- 

 toral fauna were connected with the motion of the water, we 

 ought to observe an ascent of the deep-sea fauna in quiet bays, 

 which, however, is by no means the case. But, on the other 

 hand, we must consider that the great currents of the sea 

 descend much deeper than the littoral fauna, and, indeed, the 

 Gulf-stream especially is inhabited by the deep-sea fauna in 

 its depths. 



But if it be neither the temperature, nor the chemical con- 

 ditions, nor the movements of the sea that regulate the 

 bathymetrical distribution of marine animals, there remains in 

 reality only one factor that can be taken into consideration ; and 

 this is light. 



Light is the most powerful factor amongst all the agents 

 which influence life upon the earth ; and its importance is 

 generally overlooked only because on the surface of the earth 

 it is everywhere pretty uniformly distributed, and therefore 

 gives but little occasion for the production of differences. But 

 in the sea the conditions are quite different. The light as it 

 penetrates into the water is gradually absorbed by the water ; 

 it is thus gradually changed, and finally entirely absorbed, so 

 that, at a certain depth, the sea must be perfectly dark. It 

 is to be remarked, however, that the relation of the sun's light 

 to the water of the sea is not perceptibly modified either by 

 the temperature or by any existing variations in the chemical 

 composition of the water, and that therefore this relation must 

 remain pretty nearly the same over the whole earth. Jf we 

 now place this condition vividly before us, the enormous mass 

 of the sea, above a thin illuminated zone, below a great 

 dark mass, the conviction must, to a certain extent, a priori, 

 force itself upon us, that this fundamental difference in the 

 external conditions of life must fad expression in a corre- 

 sponding difference of the living world. 



If we now consider that, according to the experiments of 

 Secchi, Pourtales, and Bouguer, the inferior limit of light in 

 the sea lies between 46 and 50 fathoms, and that this is ex- 

 actly the depth that we fixed upon at starting as the boundary- 

 line between the littoral and deep-sea faunas, there can scarcely 

 any longer be a doubt that the difference which is produced in 

 the fauna of the sea by its conditions of light is no other than 

 that which we distinguish as littoral fauna and deep-sea fauna 









